The present invention relates to the synthesis of lithiated nickel dioxide, substantially free of contaminants capable of decomposing electrochemically at cell voltages to produce gaseous decomposition products, particularly lithium hydroxide and lithium carbonate. Secondary electrochemical cells incorporating lithiated nickel dioxide as the cathode-active material are also disclosed, and in particular, lithium nickel oxide cathode active cells with carbonaceous counterelectrodes. Thermally stable lithiated nickel dioxide and methods of making same are also disclosed.
Electrochemical cells useful as electrical storage batteries usually incorporate a metal-containing anode and a cathode including an active material which can take up ions of the metal. An electrolyte incorporating ions of the metal is disposed in contact with the anode and the cathode. During discharge of the cell, metal ions leave the anode, enter the electrolyte and are taken up in the active material of the cathode, resulting in the release of electrical energy. Provided that the reaction between the metal ions and the cathode-active material is reversible, the process can be reversed by applying electrical energy to the cell. If such a reversible cathode-active material is provided in a cell having the appropriate physical configuration and an appropriate electrolyte, the cell can be recharged and reused. Rechargeable cells are commonly referred to in the battery art as "secondary" cells.
It has long been known that useful secondary cells can be made using a light alkaline metal such as sodium, potassium and particularly, lithium, as the source of the metal ions exchanged between the anode and cathode through the electrolyte. These metals are particularly useful in combination with a cathode-active material that is a sulfide or oxide of a transition metal, i.e., a metal capable of assuming plural different valence states In the past, these alkaline metals such as lithium have been used in electrochemical cells in their pure metal state as the cell counterelectrode in combination with the transition metal cathode-active material. See, for example, Dampier, J. Electrochem Soc., 121(5), 656-660 (1974). It is common knowledge that water reacts violently with alkaline metals such as sodium, potassium and lithium in their pure metal state. Not only must water be excluded from any component of a cell having an alkali metal counterelectrode, extreme care must be taken during cell assembly to avoid exposure of the counterelectrode metal material to ambient moisture and other sources of water.
Secondary lithium cell researchers have sought to develop a rechargeable lithium cell containing no metallic lithium. Cells have been developed using instead of a lithium metal counterelectrode, a intercalation host that operates near the potential of lithium, such as the material and cells incorporating same disclosed in presently co-pending U.S. Pat. Application Ser. No. 350,396 by Fong et al., filed May 11, 1989, which with the present application is commonly owned. The disclosure of which application is hereby incorporated herein by reference thereto.
Replacing lithium metal counterelectrodes with lithium intercalation host counterelectrodes removes the restrictions lithium metal counterelectrodes place upon cell design and choice of electrolytes and also the adverse effect lithium metal places upon cycling performance and safety in the finished cell. However, a source of lithium must still be supplied to the cell for exchange between the counterelectrode and cathode-active material through the electrolyte. This can be done by assembling cells with a sacrificial strip of lithium placed in electrical contact with the counterelectrode so that when electrolyte is added, the lithium is consumed by reacting with the intercalation host material of the counterelectrode. However, this wastes space and reduces cell capacity. Furthermore, while this method is advantageous to the extent that no lithium metal remains in the finished cell, the method still requires the handling of lithium metal during cell manufacture. Therefore, complicated measures are still required to prevent contact by the lithium metal with ambient moisture and other sources of water.
A preferred solution would be to use a cathode-active material which already contains the required lithium. However, many cathode-active host materials, such as MoS2, are extremely reactive when intercalated with lithium, more so than lithium metal. Lithium metal at least can be exposed to dry air for several hours because it develops protective surface passivating layers. Reactive cathode-active intercalation hosts can only be handled under inert atmospheres. This would make manufacturing more complicated than the procedures presently used with lithium metal and prohibitively expensive.
The reactivity of lithiated cathode-active intercalation hosts, however, decreases as their voltage vs. Li/Li.sup.+ increases, and at sufficiently high voltages they become air stable. Table I lists the free energy change in units of eV per Li atom, for lithium reacting with atmospheric gases, which can also be interpreted as the voltages (vs. Li/Li.sup.+) above which lithium in a cathode-active intercalation host will not react with the respective gases and hence, be air stable. The information contained in this table indicates that in order to be air stable, a lithiated cathode-active intercalation host should have a voltage of at least about three volts vs. Li/Li.sup.+. While this table indicates that for stability under carbon dioxide even higher voltages are required, at least about four volts vs. Li/Li.sup.+, ambient concentrations of carbon dioxide are relatively low and some reaction with carbon dioxide can be tolerated. However, the higher voltages providing carbon dioxide stability are also desirable as a means of increasing the energy storage capacity of the cell.
TABLE I ______________________________________ Free Energy Change (G) for Li reactions in air Reaction G (eV/Li atom)* ______________________________________ 2Li(s) + CO.sub.2 (g) + 1/2O.sub.2 (g) .fwdarw. Li.sub.2 CO.sub.3 3.82 2Li(s) + 1/2O.sub.2 (g) .fwdarw. Li.sub.2 O(s) 2.91 Li(s) + H.sub.2 O .fwdarw. LiOH(s) + 1/2H.sub.2 (g) 2.09 3Li(s) + 1/2N.sub.2 (g) .fwdarw. Li.sub.3 N(s) 1.85 ______________________________________ *for T = 25.degree. C., and partial pressures of 1 atm.
There is, however, an upper voltage limit for potential lithiated cathode-active materials, namely the maximum cathode potential that can be sustained by the electrolyte and cell hardware. At present, the hardware at cathode potential is the limiting factor. Aluminum is the most corrosion-resistant and can sustain up to 4.2 volts vs. Li/Li.sup.+. Therefore, candidate lithiated cathode-active intercalation host materials should have sufficient reversible capacity in the range of about 3 to about 4.2 volts.
Among the lithiated cathode-active materials within this voltage range is lithiated nickel dioxide. Capacity measurements over the voltage range of electrochemical secondary cells assembled with lithiated nickel dioxide indicate that this compound is a commercially feasible cathode-active material. Such a cathode-active material would be useful in both lithiumfree cells using a lithium intercalation host counterelectrode, as well as in conventional lithium cells.
Japanese published Patent Application 63-121,260 and European Patent Application Publication No. 243,926 disclose the preparation of lithiated nickel dioxide for use in lithium batteries by the solid state reaction of powdered nickel carbonates and/or oxides at temperatures in excess of 850.degree. C. in air Japanese Published Patent Application 60-74,272 discloses a nickel hydroxide coating electrochemically oxidized in a lithium hydroxide solution to obtain a "lithium doped nickel oxide" that is then heat treated at 450.degree. C. for one hour.
U.S. Pat. No. 4,567,031 discloses the preparation of lithiated nickel dioxide for use as a cathode-active material having the formula Li.sub.x Ni.sub.y O.sub.z wherein x is between 0.1 and 1.1, y is between 1.1 and 0.1 and z is between 1.9 and 2.1, by co-crystallizing or co-precipitating a stoichiometric solution of an oxygen-containing lithium salt and an oxygen-containing nickel salt. The resulting mixed salt is calcined at 400.degree.-500.degree. C. in a stream of air or a stream of carbon monoxide and carbon dioxide. The low temperature calcination is disclosed as producing a high surface area powder. Japanese Published Patent Application 63-19,761 discloses the preparation of lithiated nickel dioxide by the anodic oxidation of nickel hydroxide in an aqueous solution of lithium hydroxide. The lithiated nickel hydroxide is then washed in hot water and heated at 200.degree. C. for two hours to dry the material and drive off water to form the nickel dioxide. Lithiated nickel dioxide cathode-active material having the formula Li.sub.x Ni.sub.y O.sub.2, with x less than one and y about equal to one, is also disclosed in U.S. Pat. No. 4,302,518.
Published European Patent Application No. 345,707 discloses the preparation of lithiated nickel dioxide for use as a cathode-active material having the formula Li.sub.y Ni.sub.2-y O.sub.2 with 0.84.ltoreq.y.ltoreq.1.22, made from lithium hydroxide and nickel oxide, pulverized and mixed in stoichiometric ratio and heated in air to a temperature between 600.degree. and 800.degree. C. An excess of lithium hydroxide is used to compensate for volatilization of this material at the heating temperature. The material is disclosed as being useful as a cathode-active material for secondary cells.
Electrochemical cells having lithiated nickel dioxide as the cathode-active material typically have poor cycling capacities In addition, lithiated nickel dioxide is thermally unstable when lithium is de-intercalated upon charging of the cell. The de-intercalation forms Li.sub.1-x NiO.sub.2. As x approaches 0.5, , the nickel approaches an unstable 4+ valence, and the material releases oxygen when heated. If a charged cell is welded on the positive electrode and local heating of the Li.sub.0.5 NiO.sub.2 occurs, oxygen can be liberated in the presence of the cell electrolyte solvent, which is driven above its flashpoint, resulting in an explosion.
Even when care is taken not to thermally release oxygen from the lithiated nickel dioxide charged cells, there is a tendency for gaseous products to accumulate with cycling, leading to a hazardous pressure buildup.
A lithiated nickel dioxide cathode-active material is needed having improved cycling capacity, thermal stability, and free from the evolution of gaseous products with cycling.